1. Field of the Invention
The present invention relates to an optical scanning apparatus mounted in an image forming apparatus using an electrophotographic process technique, and an image forming apparatus comprising the optical scanning apparatus.
2. Description of the Related Art
Conventionally, an electrophotographic image forming apparatus generally causes a rotary polyhedral mirror (polygon mirror) rotated by a scanner motor to deflect a laser beam emitted by driving a semiconductor laser in accordance with input image data, thereby irradiating and scanning on a photosensitive member with the deflected laser beam. With this operation, the image forming apparatus forms a latent image, develops the latent image into a toner image, and transfers the toner image onto a recording medium, thereby forming a final image.
An image forming apparatus of this type includes optical components such as a reflection mirror and f-θ lens inserted between the polygon mirror and the photosensitive member. The f-θ lens has optical characteristics such as condensing a laser beam and compensating distortion aberration so as to guarantee temporal scanning linearity. With these characteristics, laser beams having passed through the f-θ lens are condensed and scanned on the photosensitive member in a predetermined direction at a constant speed.
However, the f-θ lens has characteristics deviating from the design values due to, for example, manufacturing variations. Therefore, color misalignment and color unevenness occur due to a registration error in write position or a magnification error, depending on the position in the main scanning direction (the longitudinal direction of the photosensitive member) of a laser beam irradiated on the photosensitive member.
That is, a laser beam passes through the f-θ lens inserted between the polygon mirror and the photosensitive member, for one main scanning interval during which one line is scanned by irradiating the laser beam on the polygon mirror mounted on a scanner motor and rotationally driven as the scanner motor rotates. Therefore, the laser beam is irradiated at a position on the photosensitive member, which has an error from an ideal image forming position due to the deviation of the characteristics of the f-θ lens. A shift from an ideal image forming position also occurs due to, for example, deterioration in the refractive index distribution or profile irregularity of an optical component upon, for example, an environmental fluctuation in the image forming apparatus.
There are proposed techniques of detecting and reducing the error from the ideal image forming position.
For example, Japanese Patent Laid-Open No. 2005-181694 proposes an arrangement which includes a mirror for reflecting and guiding a laser beam having passed through the f-θ lens to the photosensitive member, and light detectors which are arranged outside the two ends of the mirror and used to detect a sync signal serving as a write reference for a laser beam in the main scanning direction, that is, the longitudinal direction of the photosensitive member. Based on the detection signals from the light detectors, an error from an ideal time is calculated/corrected.
Japanese Patent Laid-Open No. 62-143554 proposes an arrangement which includes a beam splitter for splitting a laser beam having passed through the scanning lens into a transmitted beam component and a reflected beam component, and a detection unit for detecting scanning position arranged downstream of the beam splitter. The reflected beam component generated by the beam splitter forms an image on the surface of the photosensitive member, and the transmitted beam component is guided to the detection unit.
The arrangement which includes the light detectors arranged outside the two ends of the mirror, as disclosed in Japanese Patent Laid-Open No. 2005-181694, can detect and correct the overall error in magnification in the main scanning direction. Assume, however, that the amount of error from an ideal length from a start position of main scanning to a center position of main scanning or that from the center position to an end position of main scanning changes upon, for example, a temperature rise in a part of the lens. In this case, the error cannot be exactly detected and corrected by the technique in the above publication.
The arrangement which causes the transmitted beam component form an image on the surface of the photosensitive member and guides the reflected beam component to each detection unit, as disclosed in the above-described Japanese Patent Laid-Open No. 62-143554, can detect the scanning timing in a main scanning region. However, since the beam splitter splits the laser beam into a transmitted beam component and a reflected beam component, this detection is largely influenced by the profile irregularity of the beam splitter and the mirror vibration even when the amount of error is corrected. This makes it impossible to exactly correct the image forming position. Still worse, neither reflected light beam to scan the photosensitive member nor transmitted beam component to detect the amount of error can be ensured in sufficient amounts because the amount of light decreases by half upon beam splitting. This leads to deterioration in the detection accuracy.